Mechanical resonators are widely used in electronic oscillators as highly stable frequency references. In order to realize an electronic oscillator with a mechanical resonator as its frequency reference, the resonator needs to be engaged in a positive feedback loop consisting of amplifying circuitry with the appropriate phase shift. The same requirement applies to the emerging micro-electro-mechanical resonators requiring on-chip or in-package integration of such devices with electronic circuits in order to deliver on the promise of higher levels of integration enabled by MEMS. A variety of MEMS oscillators including both electrostatic and piezoelectric micromechanical resonators and sustaining amplifiers have been demonstrated over the past decade.